The present disclosure is related to image marking methods and apparatus, and more specifically to methods and apparatus for electrophotographic gravure printing.
Electrophotography (or Xerography) is a well-known printing technology. In one common form of electrophotography a charged receptor surface is exposed to an image to be printed. The charge on the receptor surface is modified (e.g., discharged) where it is exposed to the image. The different charge states (e.g., charged or discharged) are used to selectively retain a charged pigment material (e.g., ink or toner). For example, where the receptor surface is exposed to light and thereby discharged no pigment material remains. The pigment material remaining on the receptor surface is transferred to a desired substrate, such as paper, where it may be fused or dried on the substrate.
Generally, electrophotographic systems utilize a dry, powdered pigment material referred to as a toner. These systems generally require that the substrate be charged, and that the toner be fused to the substrate, often by heating the substrate, after transferring the toner from the receptor surface to the substrate. There is, however, a desire for methods and systems for printing with different types of surface application materials (such as inks, adhesives, surface finish treatments, protective coatings, electrically conductive regions, etc.) and on a wider variety of substrates.
For example, one common family of alternative pigment material are liquid-based inks, such as used in ink-jet and other forms of printing well-known today. In many modern printing applications, the inks used are comprised of charged pigment particles suspended in a solvent carrier.
Ink-based printing systems require relatively low viscosity inks. The viscosity of the ink affects the printing throughput, the function of transferring to and fusing the image on a substrate, the internal operations of the printing system, the cleaning of the printing system and so forth. Thus, these systems generally are limited to using inks with a viscosity of for example less than 100 centipoise (cp). However, there are many applications for which a higher viscosity ink is advantageous, such as permitting the use of a wider variety of inks and substrates, reduced cost, etc.
A number of printing techniques accommodate high viscosity inks. Gravure printing is one example of a well-known printing technology that can accommodate a relatively wider range of ink viscosities. According to this technique, an image carrier (most often a drum) is provided with a pattern of relatively very small recessed areas or cells. An ink is spread over the image carrier such that ink is retained in the cells, but not on the lands between the cells. An image-receiving substrate is brought into pressured contact with the ink-bearing plate or drum. The ink wicks out of the cells and onto the substrate, where it is dried, thereby imparting a marking onto the substrate. Gravure printing can accommodate higher viscosity inks, but the image is not variable from printing to printing—the gravure pattern is a permanent part of the image carrier.
The present disclosure is focused on a combination of electrophotography and gravure printing to obtain digital (or variable) gravure printing. There have been efforts to combine these different printing technologies. For example, WO 91/15813 (Swidler) discloses an electrostatic image transfer system by which the negative or reverse of a desired image is first exposed onto the surface of a photoreceptor, then that image is transferred to a toner roller, where the image is reversed to create the desired image on the toner roller. This image on the toner roller may then be transferred to a substrate and fused.
Another reference is U.S. Pat. No. 3,801,315. According to this reference, a gravure member is used to form an image on a substrate. The gravure member includes a number of evenly spaced cells with interstitial surface lands. A photoconductor is formed on the surface lands only (i.e., no photoconductive material within the cells). Pigment material is deposited within the cells. The photoconductor is exposed to an image, and in the regions of exposure the charge on the photoconductor is dissipated. In cells adjacent charged lands, the pigment material forms a concave meniscus, and in cells adjacent discharged lands the pigment material forms a convex meniscus, due to the electric field effects on the surface tension of the pigment material. The image is then transferred from the gravure member to a conductively backed image-receiving web brought into contact with the gravure member. Where there is a conductive difference between land and conductive backing, and the pigment material is convex within a cell, the pigment material in the cell is transferred to the receiving web. Where the meniscus of the pigment material is concave within a cell and there is no conductive difference between land and web backing, no pigment material is transferred. The image may then be transferred from the web to a substrate. However, due to the meniscus effects, and the fact that electrostatics are required to pull the pigment material out of the cells and onto the receiving web, the pigment material must be of a relatively low viscosity. Furthermore, the reference teaches using a separate photoreceptor and gravure member, requiring cleaning of the ink off of the photoreceptor for every printing pass.
Another application of electrophotography to a gravure-like process is disclosed in U.S. Pat. No. 4,493,550. According to this reference, pigment material is disposed in cells and provided with a negative charge. A positively charged photoreceptor is image-wise exposed such that certain regions are discharged and others retain the positive charge. The photoreceptor and the pigment containing cells are brought proximate one another such that the opposite charge therebetween causes the pigment material to transfer from the cells to the photoreceptor where the photoreceptor retains the positive charge but not where it is discharged. The pigment on the photoreceptor may then be transferred to substrate. Again, however, the pigment material must be of a relatively low viscosity for the electrostatic force to be sufficient to pull the pigment material from the cell to the photoreceptor. This reference also teaches using a separate photoreceptor and gravure member, requiring cleaning of the ink off of the photoreceptor for every printing pass.
An improved system and method to perform variable data printing of viscous inks would permit digital production printing in, among other fields, the commercial graphic arts and packaging markets. The ability to use viscous liquid inks would provide numerous advantages, including use of higher density/viscosity pigment, lower fixing energy (no fusing), larger substrate latitude, and lower ink spreading or dot gain. Furthermore, the ability to perform variable data printing of other surface application materials such as other forms of pigments, adhesives, surface finish treatments, protective coatings, electrically conductive regions, etc. would expand existing markets and provide new opportunities for printing materials. In general, limits on exiting printing techniques such as ink-jet printing imposed by the viscosity of printing materials can be addressed and overcome.